Volume range control in signal transmission systems



June 23,1942. v Al-muw 7 2,287,077

VOLUME RANGE CONTROL, IN SIGNAL TRANSMISSION SYSTEMS- Filed July 19,1941

FIG!

4 2 5 6 7 Twig 755mm .9 l? /3 /4 /5 s0mz 4 viz-m WAVES CARR/ER cam/5eSUPPLY SUPPLY sue s14 CAR/PIER CARR/ER SUPPLY SUPPLY INVENTOR L. G.ABRAHAM ATTORNEY Patented June 23, 1942 I VOLUME RANGE CONTROL IN SIGNALTRAN SDIISSION SYSTEMS Leonard G. Abraham, Madison, N. J., assignor toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application July 19, 1941, SerialNo. 403,135

9 Claims.

The invention relates to signal wave transmission systems andparticularly to circuits for controlling the volume range of signaltransmission over such systems.

In telephone and other signal transmission systems it is often desirableto transmit signals having a considerable range of volumes. The rangewhich can be efliciently transmitted is limited by the characteristicsof the line or other medium over which the signals are transmitted andthat of the transmission apparatus which must be employed. In order toprevent signal distortion, it is necessary that the minimum transmittedvolume level be maintained above that of the noise introduced by thetransmission medium and that the maximum transmitted volume bemaintained below that which will cause the transmission apparatus tooverload,

Rooters and squarers, which may be defined as transmission devices whichrespectively produce an output current which is proportional to a rootor a power of the input current, or other similarly reciprocalnon-linear transmission devices have been used at opposite ends of atoll telephone line or other signal transmission medium to increase theeffective signal volume range of the line. These operate by changing theshape of the signal wave at the transmitting end so as to bring itwithin the volume limits of the transmission medium and producing acomplementary distortion at the receiving end to restore the originalsignal wave form. In accomplishing this, the rooter or other distortingdevice at the transmitting end introduces certain harmonics or othermodulation products which are necessary to the proper recreation of theoriginal speech wave by the complementary apparatus at the receivingend. If the intervening medium is limited in frequency band transmitted,or if it produces delay distortion or amplitude distortion, therelations between the different components of the original wave will bedisturbed and the extra components introduced by the squarer or othercomplementary device at the receiving end which should cancel theunwanted components introduced by the rooter, may fail to accomplishthis and may even add to the undesired components so that the finalreceived wave is distorted. Tests have shown that the equality oftransmission is greatly degraded by a small amount of delay distortion.

An object of the invention is to increase the effective volume range ofa signal transmission line or other medium by the use of such deviceswithout the undesirable results referred to above.

This object is accomplished in accordance with the invention by theaddition of suitable modulating and filtering apparatus atthetransmitting end of the system properly located with respect to therooter or similar device, whereby harmonies and other modulationproducts introduced by the rooter device are effectively eliminatedbefore transmitting a rooted wave over the transmission medium, and theuse of similar apparatus suitably located with respect to the .squareror similar device at the receiving end of the line for reintroducing theeliminated components and eliminating the distortion componentsintroduced by the squarer, in order 'torecreate the original signalwave. The various objects and features of 'the invention will be broughtout in the following detailed description when read in conjunction withthe accompanying drawing in which: I i

Figs. :1 and 2 show in block schematic form different modifications ofthe invention embodied in ajsignal wave transmission system.

Atthe transmitting end of the system of Fig. 1, speech or otheralternating signalwaves are supplied from a suitable source I, which maybe a microphone or a telephone line to a carrier modulator 2 in whichthey are modulated with a carrier wave of a suitable frequency suppliedfrom the carrier wave source 3, to raise the sig-; nal frequency band toa certain higher position in the frequency spectrum. Of the products ofmodulation one side-band is selected by the bandpass filter i and theselected side-band is then passed through a rooter 5 which operates totranslate it into a .wave each of the instantaneous values of which isproportional to the square (or other) root of the correspondinginstantaneous values of the input wave. The output of the rooter 5 isthen passed through a second bandpass filter' 6 which selects the samefrequency band as the filter 4. The band of frequencies selected by thefilter 6 is then supplied to the input of a carrier demodulator 'l in'which it is modulated with a carrier wave from the carrier wave source3, as shown, or from another carrier wave source of the same frequencyto restore the signal waves to frequencies corresponding to thosesupplied by the source I. The signal output of the demodulator l is thentransmitted over a telephone or other transmission line 8.

At the receiving end of the line 8, the received signal waves aresupplied to a modulator 9 in which they are modulated with a carrierwave from the carrier wave source 10, of the same frequency as suppliedto the modulatorand demodulator at the transmitting end of the line, toraise the signal band to the same higher frequency position as the wavesin the output of the modulator 2 at the transmitting end of the line. Ofthe products of modulation, one sideband is selected by a band-passfilter l 1 identical with the filters 4 and 6 employed at thetransmitting end of the system, and the selected sideband is passedthrough a squarer I2 or other non-linear device which will change thewave shape of the supplied Wave to the reciprocal of that produced bythe rooter or other non-linear device used at the transmitting end ofthe line. The waves in the output of the squarer l2 are then passedthrough a band-pass filter I3 which selects a band having the samefrequency limits as the side-band selected by the band filter H in theinput of the squarer device. The selected band is then supplied to thedemodulator I4 in which it is modulated with a carrier wave suppliedfrom the source or other carrier wave source of the same frequency, torestore the signal waves to the same frequencies as supplied by thesource i at the transmitting end of the line, the resulting signalfrequency waves then transmitted to the subscribers station orbroadcasting station I5 having little distortion.

The rooter and squarer devices 5 and [2, respectively, used in thissystem of Fig. 1 may be a volume range compressor and expander,respectively, or any similarly reciprocal combination of non-lineardevices. They may be, for example, of the type disclosed in the patentto Crisson, 1,737,830, issued December 3, 1929, and reference may bemade to that patent for a more detailed description of the method ofoperation of these general types of devices.

The carrier supply which is indicated at each end of the system of Fig.1 as being supplied. to the modulator and demodulator in common could,of course, be separate for the two devices, thus permitting the outputof the first demodulator 1 to be at a carrier frequency suitable fortransmission over a carrier line transmission system. In fact, thedemodulator I could be eliminated entirely if the carrier frequencysupplied to the modulator 2 is selected such that the output frequenciesof the second band-pass filter 6 at the transmitting end of the line aresuitable for the transmission over the line directly. At the receivingend of the line, the modulator 9 could similarly be eliminated. If suchmodifications are not made, however, it is an advantage in having thmodulator and demodulator at the two ends of the system using the samecarrier supply since synchronism between the two carriers is thenreadily and cheaply obtained.

If the signal band to be transmitted in the system of Fig. 1 is betweensay 0 and 1 cycles, and a carrier frequency F is to be used, the bandwhich is rooted will extend from F to F f. The harmonics will be thethird, fifth and other odd harmonics of these two frequencies, and allof these and any second harmonics will be eliminated by the band filtersreadily as long as F is, say, as great as f. In addition, however, therewill be modulation products of the mA-nB form. Let f1 and f2 be the twofrequencies of interest, then the modulation products will be of thefrequencies m(F+f1 in(F+f2). The p us products of this expression willfall outside the band filter range by considerable margin. If m=n theminus products are m(,fifz) and since (fi-.I2 is. less than. I, theoutput products of the rooter will be in the band only when F mj F+j.Since such products decrease rapidly as m increases, it should besuificient to make the ratio of F/f fairly large, say, ten times.

When m=n+1, these products will be F+n(f1fz) +f1. Some of these productswill fall inside the band and might be important. However, it may benoted that in every case either 11. or n+1 must be positive. It would befeasible to design a rooter and squarer such that the even products willbe negligible and no appreciable amount of these modulation productswill fall within the band.

It appears, therefore, that the scheme of Fig. 1 will permit arooter-squarer which can use the same line frequency band as theunrooted wave with little or no increase in distortion due to the line.It should be noted that changes in net loss of the circuit will still beamplified by such a rooter-squarer, and some effects will be caused byvariations in the propagation constant of the line. However, the effectof such variations and particularly of the restricted band width will bevery greatly decreased by this action.

The modified arrangement of the invention shown in Fig. 2 differs fromthat in Fig. 1 in the following particulars. In the transmitting stationof Fig. 2 there are three parallel branch circuits A, B and C connectedbetween the output of modulator 2 and the input of demodulator 7 eachcontaining two band filters BF identical for each branch and anintermediate rooter, whereas the transmitting station of Fig. 1 has onlyone such branch. In the receiving station of Fig. 2, there are threecorresponding parallel branch circuits A, B and C connected between theoutput of the modulator 9 and the input of the demodulator 14, eachcontaining two band filters BF identical for each branch and anintermediate squarer, whereas the receiving station of Fig. 1 has onlyone'such branch.

The individual band filters BF in the inputs of the respective branchcircuits A, B and C at the transmitting station of Fig. 2 are narrowband filters passing three differing frequency ranges such as to divideone side-band component of the modulation products in the output ofmodulator 2 into three different subbands. The individual rooter in eachbranch extracts the square root of each of the instantaneous amplitudesof the particular subband selected by the preceding band filter BF, andthe second band filter BF in the output of the rooter having the samefrequency limits as the preceding filter eliminates all componentscreated by the rooter which would fall in the range of the differentsubbands selected by the corresponding band filters BF in the outputs ofthe other branches. The rooted subbands selected by the band filters inthe outputs of the three branches are combined and transmitted throughthe demodulator'7 which combines them with the carrier frequencysupplied by source 3 to restore them to the same frequency position theyhad at the input of the modulator 2, and the resulting waves afteramplification in the amplifier l6 are transmitted over th line 8 to thereceiving station.

At the receiving station the received waves are modulated in themodulator 9 with the carrier wave from the source IU of the samefrequency as supplied to the modulator and demodulator at thetransmitting stations, to raise them to the same position in thefrequency spectrum as provided by the modulator 2 at the transmittingstation. The individual band filters BF in the inputs of the parallelbranch circuits A, B and C at the receiving station, which have the samepass ranges as the corresponding filters in the corresponding branchesat the transmitting station, divide one side-band component of themodulation products in the output of modulator 9 into three difierentsubbands, which are respectively supplied to the individual squarer inthe respective branches which operates to expand their instantaneousamplitudes in the same ratio as they were compressed by the rooter inthe corresponding channel at the transmitting station to restore them totheir original amplitudes before being rooted. The band filter BF in theoutput of the individual squarer in each of the branches A, B and C,having the same pass band as the band filter BF in the input of thesquarer in the same branch, operates to eliminate any distortioncomponents introduced in the subband by the preceding squarer, whichwould fall in that of any of the subbands passed by the correspondingband filters BF in the other branches. The resulting subbands arecombined in the input of the demodulator I4 in which they are modulatedwith the carrier wave from the source ID to restore the original signalwaves Without distortion, which are transmitted to the subscriberstation l5.

If the number of parallel filter-rooter and filter-squarer branchesemployed in the system such as illustrated in Fig. 2 is madesufiiciently great, there will not be more than one distortion frequencycomponent introduced by a rooter or squarer device appearing in any onesubband at a time, so that effectively all distortion will beeliminated. Practically speaking, it is believed that two or three bands(the case illustrated) might be sufficient for this purpose,particularly if the rooting is done at carrier frequencies. One reasonfor this is that in addition to reducing the number of frequencies whichmay fall in any one rooter, breaking the band into 11. parts alsoreduces the delay distortion which might cause trouble, since only thedistortion within one band is effective in preventing propercancellation of added components. Another reason is that speech currentstend to be either mostly at one frequency or at two or threeconsiderably separated frequencies at any given instant. By properselection of a few subbands, it should be possible to guarantee thatonly quite rarely will two frequencies of strong amplitude fall withinanyone subband simultaneously.

When the bands are separated at the sending end in the system such asshown in Fig. 2, preemphasis may be introduced by putting more gain orloss in certain bands and deemphasis by th reverse action at thereceiving end.

It should be noted that the process employed in the system of Fig. 2 isnot the same thing as the usual rooting and squaring in a single band,and special consideration will need to be given to the amount of addedgain which may be used at the receiving end. For example, suppose Wehave a two-frequency wave If we transmit this normally over the line, apeak current of (A1 +A2) will usually be observed. Now with a one-bandrooter, the peak would be reduced to VA1+A2, so that at the sending endan amplification factor of g1=VA1+A2 would be allowable fort-he samepeak as before. With a multiband rooter, however, the peak would beVZ1+VZ2 and the allowable amplification factor would be im 4Z1 +1 A2 Inthe important limiting case, Where A1=A2,

g1: 2 A1 and gz l Ti which is a noise advantage reduction of threedecibels. However, in the method such as illustrated in Fig. 2, thenoise at the receiving end is generally not increased in the squarer byas much as in the simple rooter-squarer case of Fig. 1, giving a netover-all signal-to-noise advantage of three decibels or more for themethod of Fig. 2. In addition, the improvement .will tend to be greaterif cross-talk to other channels is the limiting factor rather than thepeak over-loading.

Another alternative method of obtaining the noise improvement of rootingand squaring Without any added components over the line may beillustrated as follows. Suppose that a speech wave is recorded on a filmtrack, and then the film track analyzed into its basic frequencycomponents. The produced speech current, then, will be equal to Let asecond curve with the following. equation be plotted:

Let the latter wave be converted into electrical current and amplifiedby a suitable factor. For example, let us suppose that peak overloadingdetermines the allowable line level. Then the factor will be so chosenas to give the same peak current on the line as before. Aftertransmission over the line, this wave may be broken down into itscomponents again and restored to its original form and magnitude. Anyweak noise picked up in the line will be substantially reduced comparedto the signal by this process. In addition, since only the originalsignals are present on the line, and there is no cancellation ofadditional components by the squarer, delay distortion will not degradethe quality of transmission.

Various modifications of the circuits which have been described aboveand illustrated in the drawing which are within the spirit and scope ofthe invention will occur to persons skilled in the art.

What is claimed is:

1. In a signal wave transmission system including an intermediate wavetransmission medium, the method of signaling so as to effectivelyincrease the transmission range of the system, which consists in firstmodulating a signal wave of a band of signal frequency components to betransmitted with a carrier wave of a given frequency to shift the bandto a higher position in the frequency spectrum such that one side-bandofthe modulation products is within the frequency limits of said medium,compressing the volume range of said side-band in a given ratio to bringit within the volume range limits of said system, selecting from thecompressed wave and transmitting over said medium a band of frequencieshaving the frequency limits of said side-band before compression, and ata receiving point selecting the transmitted band, producing an expansionof the volume range of the selected band which is complementary to thecompression produced at the transmitting end of said medium, selectingfrom the expanded wave a particular band having the same frequencylimits as the selected band before expansion, and modulating saidparticular band with a second carrier wave of said given frequency toreproduce the original signal wave.

2. In a signal wave transmission system including an intermediatetransmission medium, the method of increasing the effective transmissionrange of said system which consists in first modulating a signal wavecomprising a band of signal frequency components to be transmitted witha carrier wave of given frequency to shift the band to a higher positionin the frequency spectrum, selecting one side-band from the products ofmodulation, compressing the volume range of said subband in a givenratio to bring it within the volume range limits of said system,selecting from the compressed Wave a band having the same frequencylimits as said side-band before compression, modulating the resultingband with a carrier wave of said given frequency to restore eachfrequency in that band to its original frequency, transmitting theresulting modulation products over said medium and at a receiving pointmodulating the received waves with a carrier wave of said givenfrequency to shift the received band of signal frequencies to saidhigher position in the frequency spectrum, selecting a side-band fromthe resulting modulation products, producing an expansion in thatside-band which is the reciprocal of the compression produced at thetransmitting end of the medium, selecting from the expanded Wave a bandof frequencies having the frequency limits of the side-band before theexpansion, and modulating the latter selected band with a carrier waveof said given frequency to reproduce the original signal Wave.

3. A signal wave transmission system including stations connected by asignal wave transmission medium, the transmitting station including anon-linear transmission device for altering the ratio of the maximumamplitude to minimum amplitude in the signals before transmitting themto said medium, to bring them within the volume range limits of thesystem, and means for eliminating most of the distortion componentsintroduced by said non-linear device, the receiving station including areciprocal non-linear device for restoring the received signals to theiroriginal amplitude relations and means for eliminating most of thedistortion components introduced by the latter non-linear device.

4. The system of claim 3, in which the means for eliminating thedistortion components introduced by the non-linear transmission devicesat the transmitting and receiving stations comprises at each station,modulating means for translating the signal Wave input to said stationto a higher position in the frequency spectrum before transmitting itthrough the non-linear device, filtering means for selecting from theoutput of the non-linear device a band offrequencies having the samefrequency limits as the wave supplied to the input thereof, anddemodulating means for translating the frequencies in the latter.selected band to their original frequency values for transmission tosaid medium in the case of the transmitting station and for transmissionto a signal receiver in the case of the receiving station.

5. A signal wave transmission system comprising stations connected by awave transmission medium, the transmitting station comprising modulatingmeans for shifting the band of frequency components in the signal waveto be transmitted toa higher position in the frequency spectrum but suchthat the frequencies of one signal side-band of the modulation productsare within the frequency limits of the transmission medium, a.compressor for compressing the volume range of said one side-band withinthe volume range capacity of said medium, filtering means for selectingfrom the compressed wave and transmitting to said medium a band havingthe same frequency limits as the side-band in the input of saidcompressor, the receiving station comprising filtering means forselecting from the received waves the same band of frequencies selectedby said filtering means at the transmitting station, and an expander forproducing an expansion of the volume range of the selected band which isthe reciprocal of that produced by the compressor at the transmittingstation, other filtering means for selecting from the expanded wave aband having the same frequency limits as the band supplied to the inputof the expander and demodulating means for shifting the signal frequencycomponenm in the band selected by said other filtering means to theiroriginal position in the frequency spectrum to reproduce the originalsignal wave.

6. A signal wave transmission system comprising stations connected by awave transmission medium of limited volume range capacity, thetransmitting station comprising modulating means for shifting the bandof frequency components in the signal wave to be transmitted to a higherposition in the frequency spectrum, filtering means for selecting oneside-band from the resulting modulation products, a non-lineartransmission device for translating said sideband into a varyingelectrical wave the amplitude of which from instant to instant isdirectly proportional to an invariable non-linear function of thecorresponding amplitude of said sideband and the range of maximum tominimum amplitude of which is such as to bring it within the volumerange limits of said medium, a second filtering means for selecting fromthe output of said non-linear device a band of frequency componentshaving the frequency limits of said side-band, and demodulating meansfor shifting the band of frequencies in the selected band to theiroriginal positions in the frequency spectrum and supplying the resultingwave to said transmission medium, the receiving station comprising asecond modulating means for shifting the frequency of the band offrequency components received over said medium to the same higherfrequency position in the frequency spectrum to which the originalsignal frequency band was shifted by the modulating means at thetransmitting station, a third filtering means for selecting oneside-band from the resulting modelation products, a second non-lineartransmission device for producing in that selected sideband a distortionwhich is complementary to that produced by the non-linear transmissiondevice at the transmitting station, a fourth filtering means forselecting from the output of said second non-linear device a' bandhaving the same'frequency limits as that impressed on the input of saidsecond non-linear device and de-' modulating means for shifting thefrequency of the latter selected band to restore each frequency to itsoriginal frequency value to produce a wave which is a reproduction ofthe original signal wave at the transmitting station.

7. A system for effectively reducing line and apparatus distortion ofvoice frequency signals in transmission over a signal transmissioncomprising at a transmitting station a modulator for shifting the bandof voice frequencies in the signal wave to be transmitted to a higherposition in the frequency spectrum, a filter for selecting one side-bandfrom the output of said modulator, means for compressing the volumerange of said side-band to bring it within the volume range limits ofsaid system, a second filter for selecting from the output of saidcompressing means a band of frequencies having the same frequency limitsas the side-band supplied to its input, a demodulator for shifting thelatter selected band to its original voice frequency position in thefrequency spectrum, and for then supplying it to said line, and at areceiving station the same modulator-demodulator-filter combination asat the transmitting station and a volume range expander having acharacteristic which is the reciprocal of that of the compressing meansat the transmitting station, in the same relative position with respectto the latter combination as said compressing means is with respect tothe same combination at the transmitting station.

8. In a signal wave transmission system including an intermediate wavetransmission medium, the method of signaling so as effectively toincrease the transmission frequency range of said medium, which consistsin dividing a band of frequency components in a signal wave to betransmitted into a plurality of different subbands, extracting thesquare root of each of the instantaneous amplitude values of each ofsubbands, selecting from each of the rooted subbands a different subbandof frequencies having the same frequency limits as the correspondingunrooted subband to eliminate all created distortion components whichwould fall within the frequency limits of any of the other rootedsubbands, combining the resulting subbands, transmitting the resultingwave over said. medium, and at a receiving point repeating the sameprocess except that in place of the step of extracting the square rootof the subbands the reciprocal step of squaring is substituted, so as toreproduce the original signal wave.

9. In a signal wave transmission system including stations connected bya wave transmission medium, the transmitting station comprisingmodulating means for shifting a band of frequency components in a signalwave to be transmitted to a higher position in the frequency spectrum,filters for separating one side-band of the resulting modulationproducts into a plurality of different subbands, a different compressorfor compressing the volume range of each subband in the same ratio,filters for respectively selecting from the output of each compressor asubband having the same frequency limits as the corresponding subband inthe input of the compressor, means for combining the selected compressedsubbands into a common circuit and a demodulating means therein forshifting the frequency of the combination waves to restore thefrequencies therein to their original values, and for supplying theresulting wave to said medium, the receiving station comprisingmodulating means for shifting the band of frequencies in the receivedsignal wave to said higher position in the frequency spectrum, filtersfor dividing one side-band of the resulting modulation products into aplurality of different subbands, a different expander having acharacteristic which is the reciprocal of that of the compressors at thetransmitting station for respectively expanding the volume range of adifferent one of the latter subbands in the same ratio, filters forrespectively selecting from the output of each of the expanders thesubband having the same frequency limits as the corresponding subband inthe input thereof, means for combining the selected expanded subbandsinto a single circuit and demodulating means therein for restoringthefrequencies in the combination wave to their original positions inthe frequency spectrum to reproduce the original signal wave.

LEONARD G. ABRAHAM.

